Can't update iMovie in App Store

This is listed in my App Store Updates list. When I click on it, I get the message "Unable to update iMovie. To update this app, you need to accept it on the account page in the app store." Okay, so I go my account page and it's listed there, but I have idea how to "accept" it. Apple gives no help on this topic.

Any further info would be gratefully accepted!

Thank you.